A fuel cell has been proposed as a clean, efficient, and environmentally responsible energy source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for the traditional internal-combustion engine used in modern vehicles. One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to a vehicle.
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in the fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte, and thus, are directed through a load to perform work before being sent to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. Not all of the hydrogen is consumed by the stack and some of the hydrogen is output as an anode exhaust gas that may include water and nitrogen. A portion of the anode exhaust gas may be recycled to maintain an anode stoichiometry without the use of excess hydrogen.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane, for example. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture, and the membrane define a membrane electrode assembly (MEA).
Several individual fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include two hundred or more fuel cells.
Anode reactant recycling systems typically include an anode heat exchanger for heating non-recycled hydrogen to be mixed with the portion of the anode exhaust gas, a hydrogen injector, a water separator and drain valve, and an exhaust valve for removal of reactant byproducts such as water and nitrogen. Anode reactant recycling systems typically require the use of the drain valve and the exhaust valve to perform the respective tasks of removing water and nitrogen from the fuel cell stack.
Heating of the non-recycled hydrogen through the use of an anode heat exchanger is necessary prior to mixing with the recycled portion of the anode exhaust gas to militate against water condensation. Water condensation occurs when the non-recycled hydrogen (at a temperature of an operating environment the vehicle is in) is mixed with the portion of the anode exhaust gas (approximately 60°-80° C.).
Additionally, the anode exhaust gas may contain condensed water as a result of the electrochemical reaction in the fuel cell stack or lower temperatures in the anode reactant recycling system. The water condensation must then be removed by the water separator and drain valve before the mixture re-enters the fuel cell stack. Typically, the drain valve diverts the anode exhaust gas from the hydrogen injector after the anode exhaust gas passes through the water separator. Accordingly, water in the water separator is exhausted from the system with the anode exhaust gas. Water condensation not removed from the mixture that enters the fuel cell stack may cause cell starvation by inhibiting reactant flow.
Nitrogen may be removed from the anode exhaust gas by bypassing the water separator and the injector. Typically, the exhaust valve diverts the anode exhaust gas from the water separator and the hydrogen injector to an exhaust of the vehicle. Diverting the anode exhaust gas prior to the water separator and the hydrogen injector minimizes the loss of hydrogen while removing nitrogen from the anode exhaust gas.
Anode reactant recycling systems are expensive and increase a cost of a vehicle into which the anode reactant recycling system is incorporated and can be volumetrically inefficient, often requiring considerable space within an end unit of the fuel cell stack. The employment of such systems undesirably adds to a complexity in designing and manufacturing the fuel cell stack. The addition of the anode heat exchanger also undesirably requires additional componentry in order to facilitate heating the non-recycled hydrogen. Further, the use of two separate valves (the drain valve and the exhaust valve) adds undesirable cost and complexity to the anode reactant recycling system.
It would be desirable to produce an anode reactant recycling system for a fuel cell stack that eliminates the need for the anode heat exchanger, increases volumetric efficiency of the anode reactant recycling system, uses a single valve for removal of condensate and reactant byproducts from the anode reactant recycling system, and provides an upstream volume for startup pressurization.